As is known in the art, commercial and other aircraft use voice radios to communicate with ground-based air traffic control facilities. In some cases, the ground is unable to communicate with the aircraft, due to the current setting of the radios. A radio may be tuned to the wrong frequency or inadventantly have the volume turned down, or may not be selected. In the case of VFR traffic, there is no requirement to communicate with air traffic control, so it would be unlikely that a controller would know what frequency the radio was set to.
As is also known in the art, pilots are encouraged to continuously monitor an emergency frequency in addition to the frequencies that they use for normal communication. In some cases, this is not practical since aircraft may have only one radio, or may use multiple radios in a specified sequence that precludes or discourages such frequency monitoring.
Oceanic flights communicating over HF radio use SELCAL codes, i.e., tone sequences that can be used by air traffic controllers to alert an aircraft to monitor a radio frequency. While this technology may work, special equipment is required onboard the aircraft. Most aircraft do not have this equipment. Also, since this approach requires the controller to know the code for a particular flight, in many cases the controller does not know the identity of the aircraft it is desirable to contact.
A developing technology known as Automatic Dependent Surveillance Broadcast (ADS-B) is being deployed by the FAA (Federal Aviation Administration). ADS-B provides the identity of aircraft having ADS capability. ADS also provides for uplink of ‘broadcast’ products to equipped aircraft, including weather and traffic information.